The present invention relates to a radio communication system, radio communication apparatus, a radio communication method and a computer program for carrying out communications between a plurality of radio communication stations as is the case with a radio LAN (Local Area Network). More specifically, the present invention relates to a radio communication system, radio communication apparatus, a radio communication method and a computer program, in which radio communication stations make accesses to communication media while avoiding collisions in accordance with detection of carriers on the basis of a CSMA (Carrier Sense Multiple Access with Collision Avoidance) method.
To put it in more detail, the present invention relates to a radio communication system, radio communication apparatus, a radio communication method and a computer program, in which every radio communication station operates by obtaining a desired communication opportunity in accordance with the state of occupation of the communication media. More specifically, the present invention relates to a radio communication system, radio communication apparatus, a radio communication method and a computer program, in which every radio communication station operates by obtaining a desired communication opportunity by dynamically controlling a communication range in accordance with the state of occupation of the communication media.
By constructing a LAN including a plurality of computers connected to each other, information such as files and data can be shared by the computers, peripheral equipment such as a printer can be shared by the computers and information can be exchanged between the computers in the form of transmitted emails and transferred data/contents.
Traditionally, a wired LAN is generally constructed by using optical fibers, coaxial cables or twist-pair cables. In this case, a work to lay out the cables must be done. Thus, it is difficult to construct the network as a simple work. In addition, the work to draw the cables is complicated. On the top of that, after the construction, a range in which equipment can be moved from a location to another is restricted due to limited lengths of the cables so that the work to move equipment is not convenient either.
As a system for freeing the user from LAN wiring problems caused by the wire-communication method as described above, attention is paid to a radio LAN. In accordance with the radio LAN, in a work space such as an office, most of wiring cables can be eliminated. Thus, a communication terminal such as a personal computer (PC) can be moved from a location to another with ease.
In recent years, with an increased speed and decreased cost of a radio LAN, the demand for a radio LAN has been rising substantially. In particular, a study of introduction of a personal area network (PAN) is being conducted recently. The PAN is a small-scale radio network built to connect a plurality of electronic apparatus existing in the immediate surroundings of users in order to allow the electronic apparatus to communicate information between them. As the PAN, different radio systems using radio bands requiring no license from a supervisory authority are prescribed. Examples of the radio band requiring no license are the 2.4 and 5 GHz.
Standard specifications for radio network include IEEE (The Institute of Electrical and Electronics Engineers) 802.11 described in documents such as non-patent document 1, HiperLan/2 described in documents such as non-patent document 2 or 3, IEEE302.15.3 and Bluetooth communication. There are some different versions of the IEEE802.11 standard due to differences in adopted radio communication method and used frequency band. An example of the versions is an IEEE802.11a standard described in documents such as non-patent document 4.
Transmission and Reception Procedures
In general, a local area network using a radio technology is constructed by adoption of method by which an apparatus to serve as a control station called an access point or a coordinator is provided in the area covered by the local area network and a network is constructed under integrated control executed by the control station.
In a radio network including an access point, an access control method is widely adopted for transmitting information from a communication apparatus. In accordance with the access control method, first of all, a reservation is made with the access point as a reservation for a band required for the transmission of the information. Then, a transmission line is utilized in accordance with the reservation to avoid collisions with information transmissions carried out by other communication apparatus. That is to say, by providing an access point in the radio network, it is possible to carry out synchronous radio communications in which communication apparatus in the network establish synchronization with each other.
If communication apparatus on transmitting and receiver sides in a radio network including an access point carry out an asynchronous radio communication, however, the communication must be performed through the access point. Thus, the asynchronous radio communication raises a problem that the utilization of the transmission line is reduced to a half.
As another method of constructing a radio network, on the other hand, there has been proposed ad-hoc communication of allowing terminals to carry out an asynchronous communication directly. In particular, in the case of a small-scale radio network including relatively few clients located at locations close to each other, ad-hoc communication is pondered to be suitable direct asynchronous communication between any terminals without using a special access point. In a radio LAN system conforming to the IEEE802.11 standard, for example, an ad-hoc mode is provided as a mode in which the system allows terminals to operate as autonomous terminals in a distributed-processing manner and on a peer-to-peer basis without requiring a control station.
In this case, when a plurality of users makes accesses to the same channel, it is necessary to avoid contentions. As a representative communication procedure for avoiding a contention, a CSMA (Carrier Sense Multiple Access with Collision Avoidance) method is known. The CSMA method is a connection method allowing several accesses to be made on the basis of carrier detection. Since it is difficult for a terminal to receive a signal transmitted by the terminal itself as a signal representing transmitted information, in place of a CSMA/CD (Collision Detection) method, a CSMA/CA (Collision Avoidance) method is adopted. In accordance with the CSMA/CA method, a transmission of information from a communication apparatus is started only after confirming the fact that there is no transmission of information from another communication apparatus so that a collision can be avoided.
A communication based on the CSMA/CA method is explained by referring to FIG. 33 below. In the following example shown in the figure, four communication stations STA0 to STA3 exists in a communication environment.
Every communication station having data to be transmitted monitors the state of communication media for a predetermined frame interval DIFS following detection of a last packet. The frame interval DIFS is also referred to as a DCF (Distributed Coordination Function) Inter Frame Space. Then, if the communication media is clear or there is no transmitted signal during this frame interval DIFS, the communication station enters a random back-off period. If there is no transmitted signal also during this back-off period, the communication station is given a right of transmission.
In the following example shown in the figure, communication station STA0 with a set back-off period shorter than those of the other communication stations obtains a transmission right and is thus capable of starting a transmission of data to communication station STA1.
In the transmission of the data, communication station STA0 serving as the sender of the transmission of data stores information on an application of an NAV (Network Allocation Vector) and the length of time it takes to complete the transaction of the data communication in a duration field of the header of a MAC frame. In the following description, the header of a MAC frame is also referred to as a MAC header.
Station STAL serving as the receiver of the data-frame transmission carries out an operation to receive the data destined for this station itself throughout the period stored in the MAC header as a duration to be described later. As the operation to receive the data is completed, communication station STA1 transmits an ACK packet to the communication station STA0 serving as the sender of the data transmission in response to the transmitted data.
In the mean time, communication stations other than communication station STA1 serving as the receiver of the data-frame transmission to receive data frames each decode the duration field of the MAC header, recognizing that the communication media is being occupied, and inhibit a transmission of data without monitoring the media till the current transmission is ended. This work is referred to as establishment of an NAV (Network Allocation Vector). The NAV is effective throughout a time period indicated by the duration field. Typically, the duration field shows a period of time that lasts till communication station STA1 serving as the receiver of the data-frame transmission transmits an ACK packet to communication station STA0.
As described above, in accordance with the CSMA/CA method, only one communication station is capable of obtaining a transmission right while avoiding contentions and, in addition, the other communication stations each inhibit an operation to transmit data during the time period of the current data communication in order to get rid of collisions.
In this case, as is already known, a concealed-terminal problem is raised in the radio LAN network of the ad-hoc environment. A concealed terminal is a communication terminal that cannot be heard by a specific communication station serving as a communication partner in a communication between specific communication stations but can be heard by the other specific communication station serving as the other partner in the communication. Since concealed stations are not capable of carrying out a negotiation between each other, with only the CSMA/CA method described above, it is quite within the bounds of possibility that transmission operations result in a collision.
As one of methodologies for solving the concealed-terminal problem, a CSMA/CA method based on an RTS/CTS procedure is known. This known methodology is also adopted in the IEEE802.11 standard.
In accordance with the RTS/CTS method, a communication station serving as a data sender transmits an RTS (Request To Send) packet making a request for transmission of data to a communication station serving as a data receiver and the communication station serving as a data receiver transmits a CTS (Clear to Send) packet as a response to the RTS packet to inform the communication station serving as a data sender that the RTS packet has been received before the transmission of the data is started. Then, when a concealed terminal receives at least either the RTS or CTS packet, a period during which the execution of a data transmission based on the CTS/RTS procedure is expected is set as a transmission-inhibiting period of time in order to avoid collisions.
FIG. 34 is an explanatory diagram showing typical operations of this RTS/CTS procedure. In the case of this radio communication environment, however, four communication stations STA0 to STA3 are assumed to exist in the environment. Communication station STA2 is capable of communicating with adjacent communication station STA0. Communication station STA0 is capable of communicating with adjacent communication stations STA1 and STA2. Communication station STA1 is capable of communicating with adjacent communication stations STA0 and STA3. Communication station STA3 is capable of communicating with adjacent communication station STA1. In this case, communication station STA2 is seen by communication station STA1 as a concealed terminal and communication station STA3 is seen by communication station STA0 as a concealed terminal.
Every communication station having data to be transmitted monitors the state of communication media for a predetermined frame interval DIFS following detection of a last packet. The frame interval DIFS is also referred to as a DCF (Distributed Coordination Function) Inter Frame Space. If the communication media is clear or there is no transmitted signal during this frame interval DIFS, the communication station enters a random back-off period. If there is no transmitted signal also during this back-off period, the communication station is given a right of transmission.
In the following example shown in the figure, after the frame interval DIFS, communication station STA0 with a set back-off period shorter than those of the other communication stations obtains a transmission right and is thus capable of starting a transmission of data to communication station STA1.
That is to say, an RTS (request to send) packet is sent from communication station STA0 about to transmit data to communication station STA1. In response to the transmission request RTS packet, communication station STA1 transmits a CTS (clear to send) packet to communication station STA1 within a shorter frame interval SIFS (Short IFS). Then, within a shorter frame interval SIFS after receiving the CTS packet, communication station STA0 starts a transmission of data packets. As the transmission of data packets is completed, communication station STA1 transmits an ACK packet to communication station STA0 within a shorter frame interval SIFS.
At that time, communication stations STA3 and STA2 located at positions of concealed terminals seen from communication stations STA0 and STA1 respectively each execute control to inhibit a transmission of data till the communication between communication stations STA0 and STA1 described above is completed as indicated by a result of monitoring the utilization of a transmission line. To put it concretely, communication station STA2 detects the start of a data transmission from communication station STA1 serving as a data sender as evidenced by transmission of an RTS packet. Communication station STA2 then decodes information recorded in the duration field in the MAC header of the RTS packet. This information will be described later. A period, which lasts till the transmission of subsequent data packets is completed as evidenced by the transmission of the ACK packet, is recognized as a period during which the transmission line is already occupied. During this period, an NAV (Network Allocation Vector) is established and communication station STA2 enters a state of waiting for a transmission opportunity.
On the other hand, communication station STA3 detects the start of a data transmission to communication station STA1 serving as a data receiver as evidenced by transmission of a CTS packet. Communication station STA3 then decodes information recorded in the duration field in the MAC header of the CTS packet. This information will be described later. A period, which lasts till the transmission of subsequent data packets is completed as evidenced by the transmission of the ACK packet, is recognized as a period during which the transmission line is already occupied. During this period, an NAV is established and communication station STA3 enters a state of waiting for a transmission opportunity.
As described above, receiving at least either an RTS or CTS packet, a concealed terminal sets a transmission-inhibiting period of its own as a period coinciding with a time interval during which a data transmission based on the RTS/CTS procedure is expected to take place. Thus, a collision can be avoided. It is to be noted that the transmission rates for frames of exchanged CTS, data and ACK packets basically conform to the transmission rate adopted for the RTS packet.
Configuration of the Frame Format
FIG. 35 is a diagram showing a typical configuration of the frame format prescribed in the IEEE802.11a standard, which is an extension standard of IEEE802.11.
A preamble is added to the head of each packet as information indicating the existence of the packet. The preamble defines a known symbol pattern in accordance with specifications. The receiver examines the preamble of a received signal to determine whether or not the preamble matches the known symbol pattern. If the preamble of a received signal matches the known symbol pattern, a packet is determined to exist in the received signal.
A field following the preamble is defined as a signal field. The signal field includes information required for decoding an information portion of the packet. The information required for decoding the information portion of the packet is referred to as a PLCP (Physical Layer Convergence Protocol) header. The PLCP header includes a rate field, a length field, a parity bit, an encoder tail bit and a service field. The rate field shows the transmission rate of the information portion, which includes the service field of the PLCP header. The length field shows the length of the information portion. The packet receiver is capable of carrying the work to decode the information portion included in the packet as a portion following the PLCP header on the basis of results of decoding the rate and length fields included in the header.
A signal portion including the PLCP header is encoded into a noise-proof portion prior to a transmission at a speed of 6 Mbps. On the other hand, the information portion of an ordinary packet is transmitted in a transmission-rate mode having a highest possible bit rate in a range generating no errors in accordance with factors such as an SNR of the receiver.
The IEEE802.11a standard defines 8 different transmission-rate modes, i.e., modes at transmission rates of 6, 9, 12, 18, 24, 36, 48, and 54 Mbps. Thus, if the receiver and the transmitter are located at places close to each other, a transmission-rate mode at a high bit rate is selected. For a communication station existing at a far location, on the other hand, this information cannot be obtained in some cases.
The information portion is passed on to a link layer at a higher level as a PSDU (Physical Layer Service Data Unit).
As shown in FIG. 35, a preamble is included in the head of a transmitted signal as known information used for indicating the existence of the signal. FIG. 36 is a diagram showing a typical structure of the preamble as prescribed in the IEEE802.11a standard. Pre-1 and pre-2 portions of the preamble shown in FIG. 35 form a known pattern signal indicating that energy is filled for every four sub-carriers. The pattern includes a series of repeated electric waveforms each having a length of 0.8 microseconds. On the other hand, pre-3 and pre-4 portions of the preamble form a known pattern indicating that energy is filled for 52 sub-carriers. The pattern includes a series of repeated electric waveforms each having a length of 3.2 microseconds. The recipient receiving the packet is capable of determining the existence of the signal representing the packet on the basis of a result of processing a signal included in the 8.0-microsecond portion at the head of the preamble.
The IEEE802.11 standard defines some frame types. FIG. 37 is a diagram showing a typical structure of the PSDU portion in each of the RTS, CTS, ACK and data frames used in the RTS/CTS procedure described above.
The different frames each have a frame-control field and a duration field, which are defined as common fields. The frame-control field includes information indicating, among others, the type and application of the frame. To put it concretely, the frame-control field includes information described in table 1 below. On the other hand, the duration field describes information on an NAV application. To put it concretely, the duration field shows the length of time it takes to complete the transaction of all the packets.
TABLE 1Field nameLength [bits]DescriptionProtocol Version2Information on aversionType/Subtype6Identifierindicating a frametypeTo DS, From DS2Identifierindicating whateach address showsMore Fragment1Flag indicatingthe end of afragmentRetry1Flag indicatingwhetherretransmission isnecessaryPower Management1Flag showing apower managementmodeMore Data1Flag indicatingthat more data hasbeen storedWEP1Flag showingutilization of aWEPOrder1
In addition to the frame-control and duration fields described above, an RTS frame includes a receiver address (RA), a transmitter address (TA) and a frame check sequence (FCS), which is a checksum. The receiver address (RA) and the transmitter address (TA) are the address of the receiver and the address of the transmitter respectively.
In addition to the frame-control and duration fields described above, CTS and ACK frames each include a receiver address (RA) and an FCS, which is a checksum.
In addition to the frame-control and duration fields described above, a data frame includes four address fields, a sequence field (SEQ), a frame body, and an FCS, which is a checksum. The four address fields are fields for identifying, among others, communication stations serving as the sender and the transmitter. The frame body is a field containing net information to be passed on to an apparatus on a high layer.
Take the case shown in FIG. 34 as an example. In this case, communication station STA0 transmits an RTS packet to communication station STA1. The type/subtype information included in the frame-control field of the RTS packet indicates that the packet is an RTS packet. The duration field of the RTS packet shows the length of time it takes to complete the transaction to transmit all packets. That is to say, the duration field shows the length of a period ending at a time T8. The RA field describes the address of communication station STA1 and the TA field describes an address of its own.
This RTS packet is also received by communication station STA2 adjacent to communication station STA0. Receiving the RTS packet, communication station STA2 detects the preamble of the packet and then starts a work to receive an RTS signal representing the RTS packet. Subsequently, on the basis of information obtained as a result of decoding the PLCP header, communication station STA2 decodes the PSDU. Then, from the frame-control field of the PSDU, communication station STA2 recognizes the fact that the received packet is an RTS packet and transmits some information to communication station STA0 to notify communication station STA0 that the RTS packet has been received. In addition, from the RA field, communication station STA2 is capable of recognizing the fact that communication station STA2 is not a communication station for which the RTS packet is destined. Then, communication station STA2 recognizes the fact that the communication media is being occupied, establishes a NAV and inhibits transmission operations without monitoring the communication media till the present transaction started by the RTS packet is completed so that the data transmission desired by communication station STA0 is not obstructed. The established NAV is effective during a period indicated by the duration field. Thus, communication station STA2 is put in a state of being incapable of transmitting data till the time T8.
On the other hand, this RTS packet is also received by communication station STA1 designated as the destination of the packet. By execution of a procedure similar to the one described above, communication station STA1 decodes the PSDU and, from the frame-control field, communication station STA1 recognizes the fact that the received packet is an RTS packet. In addition, from the RA field, communication station STA1 is capable of recognizing the fact that communication station STA1 itself is a communication station for which the RTS packet is destined. That is to say, as communication station STA1 recognizes the fact that communication station STA0 intends to transmit packets to communication station STA1, communication station STA1 transmits a CTS packet to communication station STA0 at a time T3 within a frame interval SIFS in response to the RTS packet. The frame-control field of the PSDU of the CTS packet shows that this packet is a CTS packet. On the other hand, the duration field shows the length of time it takes to complete the transaction, that is, the length of a period ending at the time T8. The RA field describes the address of communication station STA0 serving as the destination of the packet transmission. It is to be noted that the transmission-rate mode of the CTS packet must be the same as that of the RTS packet.
By the way, this CTS packet is also received by communication station STA3 adjacent to communication station STA1. Receiving the CTS packet, communication station STA3 detects the preamble of the packet and then starts a work to receive a CTS signal representing the CTS packet. Subsequently, on the basis of information obtained as a result of decoding the PLCP header, communication station STA3 decodes the PSDU. Then, from the frame-control field of the PSDU, communication station STA3 recognizes the fact that the received packet is a CTS packet. In addition, since the RA field describes the address of communication station STA0 other than communication station STA3 itself as the destination of the packet transmission, communication station STA3 is capable of recognizing the fact that packet reception by an adjacent communication station is scheduled. Then, communication station STA3 establishes a NAV and inhibits transmission operations till the present transaction started by the RTS packet is completed so that the packet reception desired by communication station STA1 is not obstructed. The established NAV is effective during a period indicated by the duration field. Thus, communication station STA3 is put in a state of being incapable of transmitting data till the time T8.
On the other hand, this CTS packet is also received by communication station STA0 designated as the destination of the packet. By execution of a procedure similar to the one described above, communication station STA0 decodes the PSDU and, from the frame-control field, communication station STA0 recognizes the fact that the received packet is a RTS packet. In addition, from the RA field, communication station STA0 is capable of recognizing the fact that communication station STA0 is a communication station for which the packet is destined. That is to say, communication station STA0 recognizes the fact that communication station STA1 has completed preparations for receiving packets from communication station STA0. Then, communication station STA0 starts a transmission of a data packet to communication station STA1 at a time T5 within a frame interval SIFS in response to the packet.
The transmission of a data packet is ended at a time T6. If communication station STA1 is capable of decoding the data correctly, communication station STA1 transmits an ACK packet at a time T7 within a frame interval SIFS to communication station STA0. Communication station STA0 receives the ACK packet and ends the transaction to transmit and receive packets at the time T8. At the time T8, adjacent communication stations STA2 and STA3 discontinue the NAV and return to a normal state of being capable of transmitting and receiving packets.
The control and data frames are transmitted and received through the same communication media and, basically, the transmission power is not controlled.
Typical Access Control in the CSMA Procedure
The basic operations of the access control based on the CSMA method have been described by referring to FIG. 33. By referring to FIGS. 38 and 39, the following description explains operations of the access control taking the existence of a communication station into consideration.
In the example shown in FIG. 38, four communication stations STA0, STA1, STA2, and STA3 exist. Communication station STA0 intends to transmit data to communication station STA2 while communication station STA1 is transmitting data to communication station STA3. In addition, all the communication stations transmit data through common communication media.
Before communication station STA0 transmits data to communication station STA2, communication station STA0 checks the state of media utilization. While communication station STA1 is transmitting data to communication station STA3, communication station STA0 is not capable of transmitting data to communication station STA2.
In the example shown in FIG. 39, four additional communication stations STA4, STA5, STA6, and STA7 exist as well. Communication station STA4 may be transmitting data to communication station STA5, or communication station STA6 may be transmitting data to communication station STA7. In addition, all the communication stations transmit data through common communication media.
Before communication station STA0 transmits data to communication station STA2, communication station STA0 checks the state of media utilization. While communication station STA1 is transmitting data to communication station STA3, communication station STA4 is transmitting data to communication station STA5, or communication station STA6 is transmitting data to communication station STA7, communication station STA0 is not capable of transmitting data to communication station STA2.
As is obvious from the above description, as the number of communication stations transmitting data through common communication media (or transmitting data within an executable-communication range) increases, it is natural that a communication station more likely loses a transmission opportunity. The technical term ‘executable-communication range’ used in this specification means a range in which communications can be carried out.
In addition, if a specific communication station holds pieces of information on surrounding communication stations existing in an executable-reception range of the specific communication station, it is necessary to set the maximum number of surrounding communication stations whose pieces of information can be held at a large value taking a worst case into consideration so that the actual number of surrounding communication stations whose pieces of information can be held does not exceed the large value. The technical term ‘executable-reception range’ used in this specification means a range in which receptions of data can be carried out.
The following description describes a study of problems raised in an access control method based on the CSMA technique including the RTS/CTS procedure described above.
(1) Access-Control Non-Uniformity Caused by Non-Uniformity of Transmission Powers
The access control method based on the CSMA technique including the RTS/CTS procedure raises a problem of access-control non-uniformity among communication stations having different transmission powers.
In an example shown in FIG. 40, four communication stations, namely, STA2, STA0, STA1, and STA3, exist in a communication environment. It is assumed that communication station STA1 intends to transmit data to communication station STA3 and communication station STA0 intends to transmit data to communication station STA2. It is also assumed that, in the example shown in the figure, basically, only communication stations adjacent to each other are located in a range that can be reached by a transmitted electric wave. In addition, the following description also assumes transmission-power non-uniformity. Since the transmission power of communication station STA0 is smaller than the transmission power of communication station STA1, a signal transmitted by communication station STA1 to communication station STA0 in the direction of STA1→STA0 can be received by communication station STA0, but a signal transmitted by communication station STA0 to communication station STA1 in the direction of STA0→STA1 cannot be received by communication station STA1.
First of all, at a time T1, communication station STA1 transmits an RTS signal to communication station STA0 after verifying that the communication media is clear during a predetermined frame period and completing a set back-off state.
Since a signal transmitted by communication station STA1 in the direction of STA1→STA0 can be received by communication station STA0 as described above, communication station STA0 receives the RTS signal and sets a NAV ending at a time T8 at which the transaction initiated by the RTS signal will be completed. During the NAV, communication station STA0 is not capable of transmitting data. Then, in accordance with the RTS/CTS procedure described earlier, communication station STA1 transmits data to communication station STA3 and ends the transaction at the time T8.
Thereafter, assume that communication station STA0 is capable of transmitting an RTS signal at a time T9 after verifying that the communication media is clear during a predetermined frame period and completing a set back-off state in the same way as communication station STA1 did as described above. Since a signal transmitted by communication station STA0 to communication station STA1 in the direction of STA0→STA1 cannot be received by communication station STA1 as described above, the RTS signal transmitted by communication station STA0 does not arrive at communication station STA1. Thus, communication station STA1 is capable of transmitting a signal independently of the transmission/reception state of communication station STA0.
In a word, communication station STA1 capable of transmitting data with a large transmission power is placed in an unconditionally favorable position in comparison with communication station STA0, which has only a small transmission power. As a result, a problem of non-uniform accesses is raised if impartial control of accesses among communication stations is to be implemented.
It is to be noted that a variable area ad-hoc network has been proposed as described in documents such as patent document 1. In the variable area ad-hoc network, prior to a transmission of a radio packet, a transmission power required for the transmission is determined. However, every radio communication apparatus in such a network determines a transmission power by considering a frequency utilization rate and necessary transmission time for each transmission path as well as the importance and type of information to be transmitted even though the occupation state of the communication media is not taken into consideration. In other words, the communication station is not capable of recovering a transmission opportunity, which has been lost due to a particular occupation state of the communication media.
(2) Fewer Transmission Opportunities for Many Adjacent Stations
In the access method based on the CSMA technique as shown in FIG. 38, the number of transmission opportunities decreases as the number of communication stations each making an attempt to transmit and receive data through the same communication media increases. Consider a case in which a particular communication station intends to communicate with another communication station, which is located at a location extremely close to the particular communication station and has a very small loss of a pass to the particular communication station in comparison with the loss of a pass between the particular communication station and any other communication station. Even in this case, while the other communication station is receiving a signal from a further communication station, the particular communication station is not allowed to communicate with the other communication station in order to avoid a collision.
(3) Limiting the Number of Managed Stations for Many Adjacent Stations
In a radio communication system, in which every specific communication station is linked with all other communication stations in a range reachable by an electric wave transmitted by the specific communication station, for example, the specific communication station needs to store information on the other communication stations.
In most general cases, however, the number of other communication stations on which information can be stored in the specific communication station is limited by a hardware restriction of the specific communication station or a system restriction. If the number of other communication stations exceeds the limit, it is feared that complicated processing such as exception processing needs to be carried out.
As a conceivable solution to the above problem, a worst-case value or a maximum value of the number of other communication station is set at a sufficiently large value so that it is not necessary to carry out exception processing. From the implementation point of view, however, allocation of an exhaustlessly large memory area is not desirable. In addition, in dependence on the system, if too many other communication stations exist, another communication station can probably no longer be accommodated.    [List of Documents]    [Patent Document 1]
Japanese Patent Laid-open No. 2001-128231    [Non-patent Document 1]
International Standard ISO/IEC 8802-11: 1999 (E) ANSI/IEEE Std 802.11, 1999 Edition, Part 11: Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) Specifications    [Non-patent Document 2]
ETSI Standard ETSI TS 101 761-1 V1.3.1 Broadband Radio Access Networks (BRAN); HIPERLAN Type 2; Data Link Control (DLC) Layer; Part 1: Basic Data Transport Functions    [Non-patent Document 3]
ETSI TS 101 761-2 V1.3.1 Broadband Radio Access Networks (BRAN); HIPERLAN Type 2; Data Link Control (DLC) Layer; Part 2: Radio Link Control (RLC) sublayer    [Non-patent Document 4]
Supplement to IEEE Standard for Information technology-Telecommunications and information exchange between systems-Local and metropolitan area networks-Specific requirements-Part 11: Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) specifications: High-speed Physical Layer in the 5 GHZ Band